The term "post-industrial waste", as used herein, refers to plastic waste generated by a manufacturer within its facilities which may then be recycled internally by the plastic manufacturer. The term "post-consumer waste", as used herein, refers to plastic waste generated by an industrial or retail consumer of a manufactured plastic substrate.
Recycling and extending the life cycle of recycled substrates has become of the utmost importance in the packaging field. Although many streams of post-industrial or post-consumer waste plastic are recycled, some streams are difficult or impossible to recycle because of the inherent limitations in the nature of the plastics and the manner in which the plastics have been processed. For example, no efficient or economical method exists for removing ink from certain printed plastic substrates without damaging or altering the plastic itself. Particularly problematic is the removal of ink from thin film thermoplastic bags typically used in grocery stores and department stores as well as thermoplastic bags used to contain garden products, agricultural products, and animal feed products. When the ink is not removed from the bag, the recycled plastic substrate typically has an undesirable color or tint.
As government regulations and social pressure require more recycling and use of higher levels of post-consumer waste in manufactured plastic substrates, it becomes imperative to develop better recycling methods, especially, methods capable of removing printing ink from plastic substrates and methods for producing recycled plastic substrates having little or no color attributable to the printing ink.
Two general classes of synthetic polymeric resins exist: thermoplastic resins and thermosetting resins. The majority of polymeric resins manufactured are thermoplastic resins. Thermoplastic resins are high polymers that soften when exposed to heat and return to their original condition when cooled to room temperature. Natural substances that exhibit thermoplastic behavior are crude rubber and certain waxes. However, the term thermoplastic resin is usually applied to synthetic resins such as polyvinyl chloride, nylons, fluorocarbons, linear polyethylene, polyurethane prepolymer, polystyrene, polypropylene, cellulosic and acrylic resins, acetal resins, thermoplastic polyesters, acrylonitrile-butadiene-styrene terpolymers, cellulose acetate, phenylene oxide-based resins, polycarbonates, poly(methylmethacrylate), polystyrene, poly(vinylchloride), and styreneacrylonitrile copolymers.
Thermosetting resins are high polymers that solidify or set irreversibly when heated. This thermosetting property is usually associated with a cross-linking reaction of the molecular constituents induced by heat or radiation. In many cases, however, it is necessary to add curing agents such as organic peroxides or, in the case of rubber, sulfur. For example, linear polyethylene can be cross-linked to a thermosetting substrate either by radiation or by chemical reaction. Phenolics, alkyds, amino resins, polyesters, epoxides, and silicones are usually considered to be thermosetting, but the term also applies to substrates where additive-induced cross-linking is possible.
U.S. Pat. No. 3,069,308 (Lissant) discloses a method for removing ink from a paper product which comprises pulping the paper with a caustic-free aqueous solution containing an oxyalkylated alcohol.
U.S. Pat. No. 3,099,584 (Walsh) discloses a method for removing the magnetic stripe from a motion picture film which comprises agitating the film ultrasonically in a solvent selected from the group consisting of methylisobutyl ketone, butyl acetate, and 2-ethoxy ethanol.
U.S. Pat. No. 3,373,115 (Steppan) discloses solutions for removing tones from a negative-working planographic printing plate which comprises treating the plate with an organic solvent, such as ethers of ethylene glycol including ethylene glycol monoethyl ether, in conjunction with an acid having a dissociation constant greater than 10.sup.4 at 25.degree. C., and/or a salt of such an acid with a base such as ammonia, hydroxylamine, hydrazine, or an organic base having a dissociation constant less than 10.sup.3 at 25.degree. C.
French Patent publication no. 2,211,887 discloses methods for selectively removing ink from a lithographic printing plate using an aqueous solution of ethylene glycol monoethyl ether acidified with phosphoric acid. The preferred concentration range of the aqueous solution comprises from about 30% to about 50% of ethylene glycol monoethyl ether, by weight.